1. Field of the Invention
The present invention relates to a retro-focusing lens system suitable for wide angle lenses which are used in single-lens reflex cameras, digital cameras, etc., which generally use a 35 mm film or an electronic imaging device.
2. Description of the Related Art
Digital cameras or video cameras that have solid state pickup devices, such as charge coupled devices (CCDs) or complementary metal-oxide semiconductors (CMOSs), are widely in use. In particular, mega-pixel camera modules are in demand, and cameras having at least 5 million pixels and providing high definition are being developed as distribution type digital cameras. Imaging optical apparatuses, such as digital cameras or cellular phone cameras that use imaging devices such as CCDs or CMOSs, are required to be compact, light, and inexpensive.
With an increase in the demand for cameras, the demand for single-focus lenses such as telephoto lenses or wide angle lenses has increased. Retro-focusing (i.e., reversed telephoto) lenses, each having a long back focal distance and including a first lens group having a negative refractive power and a second lens group having a positive refractive power in a sequence from an object side, have been proposed as a wide angle lens. However, conventional retro-focusing lenses perform focusing by using a second lens group, and in this case the height of an extra-axial beam incident upon the lens system of a conventional retro-focusing lens greatly varies and thus aberration more frequently occurs. Therefore, conventional retro-focusing lenses require a method of correcting an aberration variation which occurs during focusing.